its like recovering the old torn up, ripped up old flap. you do the top side where the water drains over it., you are just covering the old soft material, nothing else, if you look down the vent holes on the pass side corner of the cowl, you can see the old flap, may need a flash light but its there.
look at it like a re roof, roof over another roof... or new vinyl floor over a old one, same idea.
this piece.
http://img.photobucket.com/albums/v2...r/DSC01280.jpg this flap is all that is between the rain and your fan.
i also sealed the top rubber stripping (small bead of caulk) to rule that out as well, but my flap was torn up good, covering it did the trick.